A paramedic walks by a part of fuselage at the crash site of Malaysia Airlines Flight 17 near the village of Hrabove, eastern Ukraine, Saturday, July 19, 2014. World leaders demanded Friday that pro-Russia rebels who control the eastern Ukraine crash site of Malaysia Airlines Flight 17 give immediate, unfettered access to independent investigators to determine who shot down the plane. (AP Photo/Vadim Ghirda)

An emergency worker pauses next to bodies of victims loaded on a truck at the crash site of Malaysia Airlines Flight 17 near the village of Hrabove, eastern Ukraine, Saturday, July 19, 2014. World leaders demanded Friday that pro-Russia rebels who control the eastern Ukraine crash site of Malaysia Airlines Flight 17 give immediate, unfettered access to independent investigators to determine who shot down the plane. (AP Photo/Vadim Ghirda)

KIEV, Ukraine — The Ukrainian government said Saturday that it had proof that Russia had provided the surface-to-air missile system that shot down a Malaysia Airlines passenger jet over eastern Ukraine on Thursday, killing all 298 people aboard.

That claim came as officials from Malaysia and the Netherlands pleaded for politics to be put aside so they could recover their dead, still lying in a field in a war zone.

Ukraine also accused Russia and separatist rebels in the east of trying to cover up their role by blocking recovery workers from the crash site, removing evidence and driving three missile launchers back to Russia just hours after the crash.

At a news conference in Kiev, Vitaly Nayda, the head of counterintelligence for the Ukrainian State Security Service, displayed photographs that he said showed the three Buk-M1 missile systems on the road to the Russian border. Two of the devices, missile launchers mounted on armored vehicles, crossed the border into Russia about 2 a.m. Friday, or less than 10 hours after the jet, Flight 17, was blown apart in midair, he said. The third weapon crossed about 4 a.m.

Nayda said that the missile had been fired from the town of Snizhne, in rebel-controlled territory, echoing U.S. intelligence showing the missile coming from eastern Ukraine. Both the Ukrainians and the Americans said they believed that the separatist rebels would have needed help from Russia in order to fire such a weapon.

Tensions flared on several fronts Saturday with reports of heavy fighting between rebels and government forces in the eastern city of Luhansk, a reminder that the crash site is in an active combat zone. Meanwhile, the Kremlin announced that it was imposing economic sanctions on 12 Americans in retaliation for a new round of economic sanctions announced last week against Russian companies.

The allegations of a cover-up, both to hide the weapons in the hours immediately after the missile strike and to stop investigators from collecting evidence, threatened to further inflame an already highly charged event. Officials from Malaysia and the Netherlands, which had the most citizens aboard the plane, urged that the crash site be secured and recovery operations allowed to proceed.

“We urge all those involved to respect the families and the nations who have lost their sons and daughters in this attack,” Liow Tiong Lai, the Malaysian transport minister, said at a news conference Saturday. “Yes, MH-17 has become a geopolitical issue, but we must not forget that it is a human tragedy.”

He added, “Days after the plane went down, the remains of 298 people lie uncovered. Citizens of 11 nations — none of whom are involved in the conflict in eastern Ukraine — cannot be laid to rest.”

In Kiev, officials also said there was still no clear information about the location of the flight data recorders. They were said to have been recovered at the scene but then taken by rebels.

On Friday, rebels said they had the recorders and would turn them over to international investigators. On Saturday, however, a rebel leader, Alexander Borodai, said that the devices had not yet been found, local news services reported.

The Kremlin has forcefully denied any role in the downing of the plane and has gone on the offensive, saying that the Ukrainian military’s anti-aircraft weapons may have been responsible.

Ukrainian officials called for an international investigation.

“We have proof that the terrorist attack was planned and carried out with the involvement of representatives of the Russian Federation,” Nayda, the intelligence official, said. “We know that Russia is trying to hide its terrorist activity and their direct involvement.”

Russian officials have stopped short of directly pointing a finger at Kiev, but they have issued their own calls for a thorough international inquiry.

In a statement Saturday, the Russian Foreign Ministry said it “appeals to both sides of the Ukrainian conflict, urging them to do everything possible to enable access for international experts to the airplane crash area in order to take action necessary for the investigation.”

In Malaysia, where officials are grappling with the tragedy of losing a second major jetliner this year, the government has joined the call for an investigation but is reluctant to assign blame for the crash. Experts and officials said two concerns shaped the Malaysian government’s wariness: its bruising experience with confusion after the loss of Malaysia Airlines Flight 370 more than four months ago and a desire not to alienate Russia and China, its main partner in East Asia.

The Ukrainian government charged that rebels had moved at least 38 bodies of victims to a morgue in Donetsk, a regional capital and rebel stronghold. Officials had planned to take victims to Kharkiv, a city in the east outside rebel control, where they said a special lab would help identify remains.

The Netherlands’ foreign minister, Frans Timmermans, who was in Kiev on Saturday, urged that his country be allowed to bring the bodies of victims home to their loved ones “with dignity.”

“My first priority is to return our people home,” he said after meeting with Ukrainian President Petro Poroshenko. “Families want to bury their loved ones,” he said, adding that relatives are “angered by what they hear.”

On Saturday afternoon, it was not clear who was in charge at the crash site. Journalists were restricted from entering certain areas, at times by a man in fatigues who occasionally made his point by a firing a gun into the air. He wore a badge from the general prosecutor’s office. Tents had been set up as headquarters, but emergency workers said they did not know who was in charge of plane parts.

In Kiev, Andriy Lysenko, a spokesman for Ukraine’s National Defense and Security Council, said that officials believed rebels were blocking access in order to remove missile fragments that would help prove that a Russian missile destroyed the plane. He said officials from the state Emergency Services Ministry, the Interior Ministry and the general prosecutor’s office had been denied access.

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